Chemical cross-linkers such as organic peroxides and silanes are able to link polymer chains, so as to create three-dimensional polymer networks. Cross-linking reactions lead to superior material characteristics of the polymers compared to the corresponding non-cross-linked raw polymers.
In particular, thermoplastic polyolefins and elastomers are polymers which are frequently modified by cross-linking in order to improve their rheological properties such as melt strength and their general processability and/or their final mechanical properties. The physical properties of the cross-linked polymers, for example specific molecular weight distribution, tensile and tear strength, and snappiness are generally improved.
Peroxide-based cross-linking processes are radical processes which are difficult to control and generate decomposition byproducts. Silane-based processes are two-step processes also initiated by a radical peroxide reaction to graft a silane to a polymer. The second step is then the cross-linking reaction which involves the hydrolysis of alkoxy groups.
Azide compounds, and more particularly bis sulfonyl azide compounds, are known as advantageous cross-linkers in the art.
A sulfonylazide coupling process relies on the thermal decomposition of the sulfonylazide, which generates reactive nitrenes that in the singlet state are known to undergo carbon-hydrogen (C—H) insertion to form sulfonylamido linkages. This process can be controlled by process temperature and concentration of the azide. The main advantages of sulfonylazide cross-linking versus radical cross-linking are a better processability (control of the reaction, no adverse effect on flow rate), less byproducts and a more homogeneous cross-linking of the polymers (resulting in better mechanical properties).
For example, document GB 1129399 describes the production of polymer fibers with the use of cross-linkers, such as bis azides. Sulfonyl azides and azido formates are mentioned, in particular 1,10-decane bis(sulfonazide) and 4,4′-diphenylether bis(sulfonazide).
Document U.S. Pat. No. 3,298,975 relates to partially foamed polypropylene compositions including sulfonyl azides as cross-linkers. A novel synthesis of a chloroaliphatic poly(sulfonyl azide) is described, but non-chlorinated alkane bis(sulfonyl azides) are also mentioned, such as 1,6-bis(4-azidosulfonylphenyl)hexane.
Documents U.S. Pat. No. 3,352,798 and GB 1080619 relate to the use of poly(sulfonyl azides) as cross-linkers for a broad range of polymers. Aralkylene and some sulfonyl azide compounds including ether groups are mentioned.
Documents BE 638643, BE 638644 and BE 622066 relate to the production of polypropylene foams or hollow polypropylene plastic, using sulfonyl azide or azidoformate cross-linkers similar to the ones mentioned in the other documents above.
One important disadvantage of most of the above chemical sulfonylazide cross-linking systems is that the cross-linkers are explosive. Therefore, they have to be used with phlegmatizing agents, which makes the cross-linking process more expensive. In addition, phlegmatizing agents are not easy to handle, require precise industrial adjustments, and are not desirable in every application.
For instance, DPO-BSA (4,4′-diphenylether-bis(sulfonazide)) is a well-known bis-sulfonyl azide cross-linker currently used in the industry for polymer manufacturing or post-treatment. But it has the disadvantage of being explosive according to two tests of the UN-manual for tests and criteria and the (German) explosives act (shock sensitive and explosive at heating under partial confinement in the Koenen-test). This compound needs to be phlegmatized with an inert non-explosive substance (phlegmatizing agent) for transportation, safe handling and technical use, for example in extrusion processes. A typical phlegmatized composition contains less than 67% (w/w) DPO-BSA and more than 33% (w/w) stabilizer, such as Irganox® 1010. Phlegmatizing agents have to be adjusted to the plastics manufacturing process, requiring development and testing of every new combination.
There is therefore a need for non or less explosive cross-linkers, with no need or less need for phlegmatization, such cross-linkers being advantageously synthesized in an easy way. Non or less explosive cross-linkers would also result in reduced costs of synthesis, formulation and transportation.